The invention relates to the deposition of thin layers, especially those having an interferential thickness, on transparent substrates so as to confer a particular functionality on them.
The transparent substrates may be made of an organic polymer, of a glass ceramic or, preferably, of a glass, in various applications, detailed below, of the glazing, screen or mirror type.
A recurrent problem with transparent substrates of the glass type (or with semi-transparent substrates) is that of they gradually become fouled, requiring tedious periodic cleaning. Another problem is the phenomenon of condensation, when it causes undesirable misting in contact with water vapour and, beyond simple misting, a build-up of water droplets preventing vision.
At least partial solutions have already been proposed: thus, coatings are known which are based on a fluoropolymer whose highly hydrophobic surface allows water to be rejected and less dirt to be attached. Coatings having photocatalytic properties are also known, for example those comprising anatase-crystallized titanium oxide, which are effective for degrading at least organic dirt by oxidation.
These various types of coating are effective but relatively complex. Furthermore, none of them solves all the above mentioned problems optimally. Thus, hydrophobic coatings do not prevent the condensation phenomenon and, in contrast, photocatalytic coatings are only truly effective when exposed to ultraviolet radiation and can therefore be used more outside a dwelling than inside it.
The invention therefore aims to find coatings which are simple to use and are capable of facilitating the cleaning of glass-type or similar substrates and/or of lessening the phenomenon of water vapour condensation on their surface or at the very least of preventing the condensation from resulting in the appearance of misting or of a multitude of droplets.
The subject of the invention is a transparent substrate, especially made of glass, provided on at least one of its faces with a layer based on an at least partially oxidized silicon derivative chosen from silicon dioxide, silicon oxycarbide or silicon oxynitride, and having a hydrophilic character.
Within the context of the invention, the silicon derivative may comprise only the elements Si and O in the case of SiO2, the elements Si, O and N in the case of an oxynitride and the elements Si, O and C in the case of an oxycarbide. However, the silicon derivative according to the invention also includes materials furthermore containing, in minor amounts (by weight) compared with silicon, at least one metal such as aluminium, zinc or zirconium. The addition of a metal may have three advantages. By reactive sputtering, this addition amounts to xe2x80x9cdopingxe2x80x9d the Si target in order to make it more conducting, thereby speeding up/facilitating the deposition. Furthermore, whatever the method of deposition (for example by pyrolysis), the addition of a metal of the aluminium type can increase the durability of the material, most particularly if it contains little or no carbon/nitrogen. Finally, the addition of a controlled amount of this type of metal into the layer makes it possible to vary its refractive index, especially to increase it (aluminium oxide has in an index of about 1.65, while zinc and zirconium oxides have an index of about 2).
Within the context of the invention, the silicon derivative also includes silicon oxides which are substoichiometric in terms of oxygen, of formula SiOx, where x is less than 2.
The invention has thus revealed a novel characteristic of this type of material, namely a certain hydrophilicity giving it unexpected properties: it was noticed that the substrate, preferably glass, provided with this type of layer cleans much more easily than a bare glass (less friction force for cleaning the glass with a cloth, most of the dirt being removed without any effort by spraying water). Furthermore, the rate of fouling was observed to be less, making it possible to reduce the frequency of the cleaning operations, this effect being more marked if the glass is on the outside and exposed periodically to rain: by running down the glass, rain naturally carries away the dirt. The third unexpected effect is that any water condensation phenomenon on the surface of the glass coated in this way does not reduce visibility through the glazing or reduces it very little: it seems that the water appears in the form of an invisible, transparent and homogeneous liquid film, and no longer in the form of droplets.
The same improvements are observed when comparing a glass provided with a multilayer film surmounted by the layer according to the invention with a glass provided only with the multilayer film (for example a film having a solar-control or low-emissivity function or an optical function, terminating in a layer which is chemically different from that of the invention, for example a layer of a metal oxide or metal nitride).
These advantageous effects may be adjusted/increased by varying the chemical composition, the surface appearance and the method of deposition chosen.
Thus, the layer may have a refractive index of about 1.45 (pure SiO2) or greater than 1.45 in the case of a silicon suboxide or if the derivative contains carbon or nitrogen. Advantageously, in the latter cases, the refractive index is adjusted to be between 1.45 and 1.80, especially between 1.50 and 1.75 or between 1.55 and 1.68. The term xe2x80x9crefractive indexxe2x80x9d should be understood to mean within the context of the invention either its refractive index within the usual meaning of the term when the layer is homogeneous with regard to composition and with regard to index through its thickness, or its apparent average index when the layer has a composition or an index which varies through its thickness. One advantageous embodiment of the invention relates in fact to layers whose refractive index decreases from the carrier substrate to the external surface of the layer.
There are two advantages in choosing a low refractive index:
on the one hand, the index is close to that of the glass when this is the substrate, thus preventing the glass from having a reflective appearance;
on the other hand, the more the refractive index tends to higher values, and the more the C or N content increases to the detriment of oxygen, and it turns out that the hydrophilicity of the layer is enhanced by increasing its oxygen content.
Another parameter that can influence the hydrophilicity of the layer is its surface roughness which, in certain embodiments of invention, is much higher than that of a standard bare glass.
The layer according to the invention may be deposited by any type of process capable of depositing thin layers of this type: the process may be a vacuum process such as sputtering, especially magnetic-field-enhanced sputtering (for example starting with a silicon target, optionally doped with boron or with aluminium). In order to favour the formation on the surface of Sixe2x80x94OH groups favourable to high hydrophilicity, it is possible to use a reactive atmosphere containing, for example, in addition to a purely oxidising compound of the O2 type, a compound containing hydrogen and/or to use a compound containing both hydrogen and oxygen. The reactive atmosphere may thus contain an O2/H2, O2/H2O or H2O2 mixture when a silicon oxide is manufactured. When a silicon oxynitride is to be deposited, it is possible to use reactive atmospheres comprising, as nitrogen and/or hydrogen compounds for example, an amine, an imine, hydrazine or ammonia. The SiO2 (optionally doped with a small amount of a metal or with boron)xe2x80x94based layers deposited by reactive sputtering may have quite variable refractive indices. Depending on the deposition parameters chosen, especially the pressure when sputtering the target, the refractive index (averaged between 380 and 780 nm) of the layers may thus be in the region of 1.4-1.5, resulting in quite dense layers. It may also have a lower value of about 1.25-1.40, especially 1.28-1.35, for example, around 1.30 (to within xc2x10.05). In this case, the layers are therefore less dense, with a certain amount of porosity and/or surface roughness which may favour their hydrophilicity.
Preferably, the deposition may be carried out by a sol-gel route or by pyrolysis, especially by CVD (Chemical Vapour Deposition). In the case of deposition by a sol-gel route, the sol may comprise a precursor based on tetraethyl orthosilicate TEOS and be deposited by known techniques such as dipping, spray coating or spin coating, or else the deposition method referred to as flow coating. In the case of deposition by CVD, a silicon precursor in the form of an SiH4 type silane may thus be used. The silicon precursor may also be an organosilane, of the RSiX3 type, where X is a halogen of the chlorine type and R is an alkyl (which is linear or branched, having, for example, from 1 to 10 carbon atoms or more). It may be an organosilane of the RySiX4-y type, with the same conventions regarding R and X, or a compound belonging to the family of ethoxysilanes. Other gases/precursors may be added to the silicon precursor(s), such as ethylene or a nitrogen-containing derivative such as ammonia or an amine (especially a primary amine). An oxidising agent (O2, H2O, H2O2, etc, )may also possibly be present.
It has also been noticed that a certain amount of surface roughness in the layer favours the above mentioned beneficial effects thereof, which roughness can be controlled especially by the parameters governing the deposition of the layer and by the actual preparation of the surface on which the layer proper is deposited.
The measured contact angle between water and the coatings according to the invention is advantageously less than 35xc2x0, or less than or equal to 25xc2x0, for example between 15xc2x0 and 25xc2x0: this actually denotes hydrophilicity (to be compared with the contact angle on standard bare glass which is in general 40xc2x0). This is not necessarily a very high degree of hydrophilicity which results in the beneficial effects of the invention, or is even modest hydrophilicity, but, being significantly greater than that of bare glass, it is effective. The condensation phenomenon is not necessarily eliminated, but it does prevent the appearance of drops (in fact when the contact angle is less than 7xc2x0 or 10xc2x0, the mist becomes invisible, even though condensation is still present).
According to certain embodiments, and especially in the case of layers deposited by CVD, the contact angle may be less than 15xc2x0, and even especially less than 10xc2x0.
The layer according to the invention may have a chemical composition which varies through its thickness. Advantageously it may have an oxygen concentration increasing towards its xe2x80x9cexternalxe2x80x9d surface (that is to say its surface furthest away from the carrier substrate). It is thus possible to have a silicon oxycarbide or oxynitride layer which is markedly richer in C or N near its surface closest to the substrate, and richer in O near its external surface, as far as even forming an almost pure (thin) layer of SiO2 on top of a layer having a chemical composition richer in C or in N; or even a layer of almost pure Si or Si3N4. This oxygen concentration gradient may be obtained by adjusting the deposition conditions or by surface oxidation after deposition, for example by a heat treatment.
A high oxygen concentration on the surface of the layer is in fact favourable in the sense that it results in a high content of hydroxyl bonds Sixe2x80x94Oxe2x80x94H on the surface, making it hydrophilic.
The layer according to the invention preferably has a thickness of at least 5 nm, especially between 5 and 100 nm, for example between 10 and 60 nm.
The layer of the invention may form part of a film of thin layers, by being the final layer of the film (or an additional layer to a given film), this layer being the furthest from the substrate. It may, for example, be an antireflection film (an alternation of layers with a high refractive index and layers with a low refractive index, such as TiO2/SiO2/TiO2/layer according to the invention, the TiO2 possibly being substituted with Nb2O5, Si3N4, SnO2, etc). It may also be a film of the solar-control type, such as a film of the type consisting of an optional sublayer/TiN/layer according to the invention or a solar-control layer based on TiO2 or a mixed iron cobalt and chromium oxide: glazing thus coated is sold by Saint-Gobain Glass France under the name xe2x80x9cVision-Litexe2x80x9d, xe2x80x9cStarxc3xa9lio and xe2x80x9cAntxc3xa9lioxe2x80x9d respectively. It may also include multilayer films comprising at least one silver-based layer having a low-emissivity or solar-control function (glazing thus coated being sold by Saint-Gobain Glass France under the name xe2x80x9cPlanithermxe2x80x9d), or low-emissivity films whose functional layer is based on fluorine-doped tin oxide (glazing thus coated being sold by Saint-Gobain Glass France under the name of xe2x80x9cEKOxe2x80x9d glazing), or else solar-control films whose functional layer is based on steel or an Ni/Cr alloy (glazing thus coated being sold by Saint-Gobain Vitrage under the name of xe2x80x9cCool-Litexe2x80x9d glazing). For further details, the reader may refer to the patents EP-638 528, EP-718 250, EP-511 901, EP-728 712, W097/43224, EP-638 527 and EP-573 325.
When the substrate is made of glass, it may be curved and/or toughened or annealed before or after deposition of the layer or layers.
The subject of the invention is also the application of the substrates described above to the manufacture of glazing with an xe2x80x9canticondensationxe2x80x9d effect and/or an xe2x80x9cantisoilingxe2x80x9d effect and/or easily cleanable glazing (within the context of the invention, xe2x80x9canticondensationxe2x80x9d means that there may be condensation, but it entails none or few of the negative consequences on visibility through the glazing). The glazing may be for buildings, for vehicles, for mirrors, and most especially for bathroom mirrors, rear view mirrors, shower cabinet glass, glazed doors and internal partitions, urban furniture, display panels and display screens of the television or computer screen type.